This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-215177, filed Jul. 29, 1999; and No. 11-215178, filed Jul. 29, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a cleaning method and a cleaning apparatus for an integrated photovoltaic module for converting light energy such as solar energy into electrical energy.
A thin-film photovoltaic module that is used in a photovoltaic apparatus of the integrated thin film type comprises a transparent first electrode layer formed on a glass substrate, an amorphous semiconductor layer, a second electrode layer, etc. Manufacturing the photovoltaic module does not require much material, and an integrated photovoltaic element with a large area can be formed directly on the substrate, so that the manufacturing cost can be lowered.
The photovoltaic module is manufactured by a method that includes a step of thin film deposition, such as CVD (chemical vapor deposition) or sputtering, and a patterning step based on laser scribing. In a typical integrated photovoltaic module, a large number of photovoltaic cells, which are connected electrically in series with one another, are formed on a glass substrate. A large-area substrate, measuring 90 cm by 45 cm, for example, is used in a photovoltaic module for electric power that is installed outdoors.
A thin-film photovoltaic module shown in FIG. 7A comprises a glass substrate 2 for use as an insulating substrate and photovoltaic cells C formed on the substrate 2. Each photovoltaic cell C includes a first electrode layer 3 formed on the substrate 2, a semiconductor photovoltaic layer 5 formed of amorphous silicon or the like, and a second electrode layer 7. In the description to follow, a laminate that includes the first electrode layer 3 formed on the substrate 2, photovoltaic layer 5, and second electrode layer 7 will be referred to as a laminate L (shown in FIGS. 7A and 7B).
In general, a transparent conductive film of tin oxide (SnO2), zinc oxide (ZnO), or indium tin oxide (ITO) is used for the first electrode layer 3, while a metallic conductive material such as silver (Ag), aluminum (Al), or chromium (Cr) is used for the second electrode layer 7. The adjacent photovoltaic cells C are connected electrically in series with one another by means of the conductive material that fills grooves 6 for series connection.
The photovoltaic module 1 is manufactured in the following manner. First, the transparent conductive film is deposited as the first electrode layer 3 on the glass substrate 2. Then, grooves 4 are formed in the first electrode layer 3 by laser scribing, in order to divide the layer 3 into a plurality of regions corresponding to the photovoltaic cells C. The scribed grooves 4 extend straight at right angles to the drawing plane of FIG. 7A. The semiconductor photovoltaic layer 5 of amorphous silicon, which includes a p-i-n junction, is deposited on the first electrode layer 3 by plasma CVD. The grooves 6 are formed in the photovoltaic layer 5 by laser scribing, whereby the adjacent photovoltaic cells are connected electrically in series with one another. The grooves 6 also extend straight at right angles to the drawing plane of FIG. 7A. Subsequently, the second electrode layer 7 of metal such as Ag, Al, or Cr is formed to cover the photovoltaic layer 5. The grooves 6 are also packed with this metal. Further, grooves 8 are formed in order to divide the photovoltaic layer 5 and the second electrode layer 7 into a plurality of regions corresponding to the photovoltaic cells C. The grooves 8, which are also formed by laser scribing, also extend straight at right angles to the drawing plane of FIG. 7A. Preferably, the grooves 8 are deep enough to reach the first electrode layer 3.
[First Problem]
In some cases, particles, such as swarf, burrs, etc., may be produced in and around the grooves when the grooves are formed in the aforesaid manner by laser scribing. If the particles are left in the photovoltaic module 1, the photovoltaic cells C are electrically shorted, thus resulting in lowering of the output, insulation, and withstand voltage characteristics of the module 1.
As is described in Jpn. Pat. Appln. KOKAI Publication No. 7-79007, therefore, a proposal has conventionally been made to apply a laser beam from a glass substrate side in order to reduce swarf or burrs that are produced during the laser scribing process. Further, back-reflection electrode processing using the fourth harmonic of a YAG laser is described in Jpn. Pat. Appln. KOKAI Publication No. 10-242489. According to a known technique described in Jpn. Pat. Appln. KOKAI Publication No. 9-8337, moreover, a cleaning process is carried out after grooves are formed by laser scribing. According to another technique described in Jpn. Pat. Appln. KOKAI Publication No. 60-110178, furthermore, a photovoltaic module is subjected to ultrasonic cleaning in a cleaning fluid after grooves are formed by laser scribing.
According to the technique described in Jpn. Pat. Appln. KOKAI Publication No. 7-79007, burrs can be only reduced and not removed. Besides, groove working is slow and inefficient. According to the technique described in Jpn. Pat. Appln. KOKAI Publication No. 10-242489, the laser power lacks in stability, and the groove working speed is low. According to the techniques described in Jpn. Pat. Appln. KOKAI Publications Nos. 9-8337 and 60-110178, ultrasonic waves must be applied for a long time to remove particles deep in the grooves.
[Second Problem]
As the electrode layers 3 and 7 of the photovoltaic module 1 are formed, some of the conductive material for the layers 3 and 7 sometimes may get to the end faces and under surfaces of the substrate 2. Although the individual cells C are separated from one another on the substrate 2, in this case, they inevitably conduct to one another by means of the conductive material that adheres to the end faces and under surface of the substrate 2. This results in lowering of the output characteristics of the photovoltaic module 1.
To solve this problem, grooves 9 for insulation are formed on the peripheral edge portion of the photovoltaic module 1, as shown in FIG. 14. The grooves 9 serve electrically to separate a power generating region G, which includes the cells C and the groove 8, from its peripheral regions 10. The grooves 9 are formed covering the whole periphery of the module 1 by laser scribing. With use of the grooves 9 formed in this manner, the cells can be prevented from being short-circuited by the conductive material that adheres to the ends faces and under surface of the substrate 2.
In general, the width of each groove 9 that is formed by laser scribing ranges from about 0.05 mm to 1.0 mm. After the grooves 9 are formed, a cover layer of an electrical insulating material is formed on the second electrode layer 7. Before this cover layer is formed, a cleaning process for cleaning the photovoltaic module 1 is carried out. Before or during the cleaning process, the module 1 generates electric power to produce electromotive force as it receives surrounding light. Since the electromotive force of the photovoltaic cell C is at about 0.85V, the potential difference between cells C1 and C2 on the positive- and negative-electrode sides shown in FIG. 14 comes up to about 53V in the case of the module 1 in which 63 cells C, for example, are connected in series with one another.
Let it be supposed that a waterdrop W1 adheres to a part 9a of a groove 9, as shown in FIG. 14, with the potential difference thus maintained between the two electrodes in the cleaning process. In this case, the cell C to which the waterdrop W1 adheres and the peripheral regions 10 are electrically shorted so that their potentials are equal. In consequence, the cell C1 that is situated on the positive-electrode side of the cell C to which the waterdrop W1 adheres becomes higher in potential than the peripheral regions 10. The cell C2 that is situated on the negative-electrode side of the cell C to which the waterdrop W1 adheres becomes lower in potential than the regions 10.
Let it be supposed that new waterdrops W2 and W3 stick to the part 9a of the groove 9, as shown in FIG. 14, with the potential difference maintained between the cells and the peripheral regions 10. In this case, current flows form the low-potential side to the high-potential side between the peripheral regions 10 and the cells C between which the potential difference exists. As shown in FIG. 15, for example, therefore, a migration of a metal (e.g., silver) develops in a conduction path (short circuit) that is formed in the part 9a of the groove, and a dendrite (e.g., crystal of silver) K grows. When the waterdrops form an ion path, the speed of growth of the silver migration is approximately 0.1 mm/sec if the rate of electrolysis is at hundreds of volts per millimeters. Thus, in the cleaning process for the photovoltaic module 1, the metal migration develops in the grooves 9, and insulation failure occurs between the power generating region G and the peripheral regions 10.
In an actual cleaning process, the photovoltaic module 1 is carried into a cleaning chamber by means of a conveyor. At the start of cleaning, therefore, drops of water for use as a cleaning fluid adheres to the front of the module 1. The waterdrops cause the aforesaid migration. In the cleaning chamber, the module 1 is covered entirely with water, so that all the cells share the same potential. The speed of growth of the metallic crystal by the migration is so high, however, that a short circuit attributable to the migration is formed before all the cells reach the same potential.
In order to solve the problem of the migration, the inventor hereof proposed migration preventing means described in Jpn. Pat. Appln. KOKAI Publication No. 10-209477. In a photovoltaic module cleaning process, according to this arrangement, cells on the positive- and negative-electrode sides, among series-connected cells, and peripheral regions are shorted by means of a conductor. The conductor is removed after the cleaning process is finished. However, this migration preventing means requires laborious operations, including installing the short-circuiting conductor before cleaning the photovoltaic module, removing the conductor after the cleaning, etc. Thus, there is room for improvement.
A cleaning method according to the present invention includes a process for transporting a photovoltaic module having scribed grooves and immersed in a cleaning fluid and applying ultrasonic vibration to the cleaning fluid. Another cleaning method according to the invention includes a process for bringing a rotating brush into contact with the surface of a laminate of a photovoltaic module and blowing pressurized air against the laminate simultaneously. Still another cleaning method according to the invention includes a process for blowing or spraying a fluid, such as pressurized water or pressurized air, along scribed grooves of the photovoltaic module by means of nozzles while moving the nozzles in parallel and relatively to the scribed grooves. Particles that are produced in the photovoltaic module by laser scribing can be efficiently removed by these cleaning methods.
A cleaning apparatus according to the invention includes a reserver tank containing a cleaning fluid, a conveyor extending through the reserver tank and capable of transporting a photovoltaic module in a manner such that the module is immersed in the cleaning fluid, while being kept in a horizontal position with the laminate upward as it is transported, and an ultrasonic vibrator for applying ultrasonic vibration to the cleaning fluid. As the photovoltaic module is transported, this cleaning apparatus applies ultrasonic vibration to the module in the cleaning fluid, thereby subjecting the module to ultrasonic cleaning. Preferably, the output of the ultrasonic vibrator ranges from 0.2 W/cm2 to 1.0 W/cm2.
Another cleaning apparatus includes a platform for supporting a photovoltaic module with its laminate upward, a vibration generator for applying vibration to the platform, and a jet nozzle mechanism opposed to the top surface of the photovoltaic module and capable of blowing a fluid, such as pressurized air, along scribed grooves by means of a plurality of air nozzles which are located to correspond individually to pitches of the scribed grooves while moving the air nozzles in parallel and relatively to the scribed grooves. This cleaning apparatus applies vibration to the photovoltaic module as its blows the fluid along the scribed grooves.
According to the cleaning apparatus of the invention, particles, such as swarf, burrs, etc. that are produced by laser scribing in the process of manufacturing the photovoltaic module, can be securely removed in a short time. According to this cleaning apparatus, photovoltaic modules with high output, insulation, and withstand voltage can be obtained. In this cleaning apparatus of the present invention, a large number of photovoltaic modules can be efficiently cleaned, since they are continuously cleaned during transportation in a plant. Since each photovoltaic module moves over an ultrasonic vibrator as it is cleaned, ultrasonic vibration can be applied substantially uniformly to the whole module.
A cleaning apparatus according to the invention includes a conveyor for transporting a photovoltaic module in a direction associated with the scribed grooves, a spraying mechanism for spraying a cleaning fluid against the photovoltaic module being transported, and electrical contact means extending across grooves, which divide a power generating region and peripheral regions of the module, and capable of electrically conducting at least those ones of photovoltaic cells which are situated on the positive- and negative-electrode sides of the power generating region and the peripheral regions during the transportation of the photovoltaic module. An example of the electrical contact means includes a conductive supporter ranging from the power generating region and the peripheral regions of the photovoltaic module and a conductive brush provided on the conductive supporter and capable of touching the photovoltaic cells in the power generating region and the peripheral regions.
In the cleaning apparatus of the invention, the cleaning fluid from the spraying mechanism is sprayed against the photovoltaic module to clean it while the module is being transported by means of the conveyor. The power generating region and the peripheral regions of the module can be kept at the same potential by being shorted by means of the electrical contact means. Accordingly, a potential difference can be prevented from developing between the cells and the peripheral regions before the photovoltaic module is covered entire with the cleaning fluid. Thus, there is no possibility of a short circuit being formed in the grooves by means of a metal migration. Further, there is no need of laborious operations, such as attaching to or removing a conductor for adjusting the cells to the same potential from the photovoltaic module before or after the cleaning operation.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.